I am myopic & I use glasses of no. 2.0 but II don't wear them all the time, only during lectures. Wil it weaken my eyesight more?

No it should not. It may even be better to do your reading and studying without them as it relaxes your accommodation and allows you to use your nearsightedness to read.
Not at all. Contrary to popular belief, glasses do not weaken the eyes; all they do is improve the vision one has by providing the focus that the eyes are missing. The eyes can naturally become more nearsighted or farsighted over time, and glasses simply help to correct the vision that you have.